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Wednesday, February 2, 2011

Lithium Battery Charging

Prof Dan Hammerstrom of Portland State U. to OEVA

For Lithium batteries (the Leaf uses AESC's patented laminated lithium ion batteries) the lifetime of the battery is directly related to current rates into and out of the battery as well as how often it is deeply discharged.

Batteries store charge by undergoing fairly complex chemical interactions, each time they charge and discharge there is slight degradation to the molecular structure of the battery. Lithium ion batteries operate by the inter-calation of lithium ions into the molecular structure of an electrode, which involves even more complex chemistry.

I couldn't find any lifetime numbers for the AESC batteries, but as of a few years ago, a typical Li-ion battery would exhibit serious degradation after a few thousand deep discharge / charge cycles.

Lithium battery chemistries are getting better all the time. But there is no question that fast charge is hard on batteries, you are putting huge amounts of current into the battery in a relative short time. Batteries have fairly high resistance, so there are thermal effects too. Unlike the Volt and the Tesla which liquid cool their batteries, the Leaf uses simple air cooling (a fan). So there is the possibility of elevated temperature during fast charge which can also reduce lifetime. (Be glad you are not fast charging your Leaf in August in Pheonix.)

If your driving patterns involve regular fast charge, then you will most likely reduce the lifetime of your batteries, the question is by how much. I'll keep looking for lifetime numbers for the AESC's cells and report back to you all if I find anything (some of you may be able to find such information too). As people have more experience with the Leaf, those numbers will become available.

As Mike Butts points out, it is ironic that the Tesla, which has a huge battery system, probably rarely fully discharges the pack and therefore has less need of fast charging, which means that it will probably have the fewest problems and longest battery lifetime. So the most expensive car will probably have the lowest battery replacement rate.

BTW, I have a friend who continues to "deep discharge" his cell phone and laptop to avoid "memory" effects. Li-ion batteries have very little memory effect (unlike Ni-Cad batteries), so my friend is basically just dramatically reducing the lifetime of these batteries for no good reason.